1. Technical Field
The present invention relates to conduits that are useful for transporting fluids. More particularly, the present invention relates to the heating of such conduits to maintain the contents thereof at a constant and/or elevated temperature.
2. Description of the Prior Art
During transport through a conduit, the contents of the conduit may be subjected to a wide range of temperatures, depending upon conduit location (e.g. controlled or ambient environmental conditions), length of the conduit, fluid pressure within the conduit, etc. These variations in temperature can affect the contents of the conduit, such that a vapor component of the conduit's contents may condense, e.g. water condensation may occur.
Such vapor condensation is highly undesirable in most fluid transport applications. For example, the liquid may freeze and disrupt fluid flow within the conduit or even rupture the conduit, causing untold waste and creating serious environmental and health hazards. The liquid or partially frozen liquid may also be transferred through the conduit and into processing equipment, adversely affecting equipment operation, contaminating process constituents, or even damaging the equipment and/or work in progress.
To avoid the problems associated with vapor condensation, it is a common practice in the semiconductor industry, for example, to heat the pipes that are used to transport certain gases to and from processing equipment. The use of heat prevents water vapor from condensing or fleezing, and other undesired effects associated therewith. For example, when water vapor is allowed to condense inside a mass flow controller (MFC) the metering accuracy of the controller is adversely affected. In such event, normal operation of the controller may only be obtained after the MFC is dried out.
Such fluid heating is also helpful to maintain the fluids at a proper temperature, such that state conversion, e.g. from gas to liquid, does not occur. For example, the chemicals that are used in semiconductor processing, including water, may be less hazardous or corrosive in a gaseous form than in a liquid form. For example, BBr.sub.3 is corrosive when in liquid form but not corrosive in gaseous form. Thus, it is beneficial to maintain such chemicals at an elevated temperature, at a corresponding unsaturated vapor pressure.
The state of the art for heating a conduit such that its contents are maintained at a constant and/or elevated temperature consists of a heating cable or ribbon, such as is manufactured by the Chemelex Division of Raychem Corporation, Menlo Park, Calif. An example of such prior art heating ribbon is shown schematically in FIG. 1.
The heating ribbon 10 is formed with a conductive polymer in which a self-regulating polymeric heating element 16, made of a polymer mixed with conductive carbon is formed between parallel conductive bus wires 12, 14. This structure is maintained along an entire ribbon length.
Such prior art heating ribbon is often wrapped in an electrically insulating sheath 18. As electrical insulation is usually thermally insulating there is typically some loss of efficiency in heat transfer from the heating ribbon to the conduit. A thermally insulating sheath may also be installed around the conduit, as well as the heating ribbon to prevent heat loss and thus improve heat transfer efficiency.
Such heating ribbons are moderately compliant and flexible and therefore may be wrapped around a conduit or run along the length of a conduit in a manner that somewhat conforms to the shape of the conduit. The heating ribbons are attached to a conduit by special fasteners (tie-wraps, wire, etc.), one placed every few inches, followed by a combination of insulation and metal tape. The heating ribbons are readily powered by any convenient source, usually an AC main power source.
One problem often encountered with known heating ribbons is that of providing uniform heating along a long run of conduit, such that temperature gradients which give rise to vapor condensation or phase conversion are avoided. It has been found in practice that long runs of such ribbons may not consistently contact the conduit's outer surface and, therefore, do not provide uniform and consistent transfer of heat to the conduit. Also, at times the installer may run the heating tape on top of the conduit in lieu of the bottom. This adversely affects heat transfer. Thus, hot or cold spots may result, often exacerbating the problem of vapor condensation or phase conversion. Such inefficient transfer of heat to a conduit also significantly increases the cost of heater operation, and thereby increases overall processing costs.
It is generally recognized that known conduit heating devices, such as the heating ribbon described above, are often difficult to install due to their stiffness. This is particularly true when considering the varying levels of skill different installers possess and the resulting variations in quality of installation. For example, during installation the heating ribbon is often seriously deformed or cut at a conduit corner coupling such that a hazardous condition, as could result in shorting or breach of insulation integrity, is created. That is, the design and manufacturing of ultra clean systems uses weld fittings which have sharp edges. Typical of such fittings are the CAJON MICRO-FIT.RTM. weld fittings, supplied by CAJON Company of Macedonia, Ohio. Such heating devices are also difficult to maintain.